The invention relates to a switching circuit comprising: a signal terminal for receiving a signal voltage; a reference terminal; a first transistor having a base, an emitter connected to the signal terminal, and a collector connected to a node; a second transistor of the same conductivity type as the first transistor and having a base, an emitter connected to the reference terminal, and a collector connected to the node; control means coupled to the base of the first transistor and to the base of the second transistor to turn on and turn off the first and the second transistor in response to a control signal so as to obtain a closed state and an open state, respectively, of the switching circuit.
Such a switching circuit is known from European Patent Application EP 0 091 119. Such switching circuits serve to short-circuit the signal voltage on the signal terminal to the reference terminal. A coupling capacitor connects the signal terminal to a signal source, as a result of which an a.c. signal appears on the signal terminal. This a.c. signal is short-circuited to the reference terminal under command of the control signal in the closed state of the switching circuit. For audio signals the switching circuit functions as a mute switch under control of a mute signal. For video signals the switching circuit operates as a clamp switch in response to a clamp signal. In the known switching circuit two transistors of the same conductivity type are arranged in series between the signal terminal and the reference terminal. The first transistor operates in the reverse mode and has its emitter connected to the signal terminal. The second transistor operates in the forward mode and has its emitter connected to the reference terminal. The collectors of the two transistors are connected to one another in the node. The control means include current sources which in the closed state of the switching circuit supply bias current to the bases of the two transistors under command of the control signal. The two transistors then form a low-impedance current path between the signal terminal and the reference terminal, as a result of which the signal voltage is short-circuited to the reference terminal. The known two-transistor series arrangement is better suitable for larger signal voltages than arrangements comprising only one transistor, which may or may not operate in the reverse mode, between the signal terminal and the reference terminal.
In the integrated circuits the reference terminal is often connected to the substrate of the semiconductor body of the integrated circuit. When it is assumed that the first and the second transistor are NPN transistors in a P substrate there will be a parasitic PNP transistor having its base, emitter and collector connected to the node between the collectors of the first and the second transistor, the reference terminal or substrate, and the base of the first transistor, respectively. The parasitic transistor and the first transistor form together a thyristor which is fired automatically when the signal voltage on the signal terminal is sufficiently negative. The result is that the signal terminal is short-circuited directly to the reference terminal or the substrate. This effect is undesirable if the switching circuit should be open-circuited, i.e. in the open state of the switching circuit when the two transistors must be cut off. The spontaneous firing of the thyristor consequently imposes a limit on the permissible amplitude of the signal voltage on the signal terminal.